1. Field of Invention
The present invention relates to a seal roll apparatus for preventing atmospheric gases from leaking out of the heat treatment zone in a continuous heat-treatment furnace for continuously heat-treating a workpiece strip, such as a steel strip or an aluminum strip, and to a sealing method.
2. Description of Related Art
A continuous heat-treatment furnace basically includes a heating zone in which a workpiece strip is heated to a predetermined temperature for annealing treatment and a cooling zone in which the annealed workpiece is cooled down to room temperature. FIG. 3 is a diagram showing a typical example of a continuous heat-treatment furnace for a cold-rolled steel strip. The following elements are disposed in the continuous heat-treatment furnace in the following order: a pre-heating section 15 for pre-heating a steel strip S by recovering and heat-exchanging usable heat of exhaust gases from a heating section 14, the heating section 14 for heating the steel strip S to a predetermined temperature, a soaking section 16 for soaking the steel strip S which has been heated to the predetermined temperature, a slow-cooling section 17 for slow-cooling the soaked steel strip S, a rapid-cooling section 18 for rapid-cooling the slow-cooled steel strip S, an over-aging section 19 for over-aging treatment, and a final cooling section 20 for finally cooling the steel strip S down to room temperature. The pre-heating section 15, the heating section 14, and the soaking section 16 constitute a heating zone, and the slow-cooling section 17 through the final cooling section 20 constitute a cooling zone.
The cold-rolled steel strip S which has been subjected to work hardening is passed through the individual treatment sections by a hearth roll 21 mounted in the furnace. When a workpiece that does not require over-aging treatment is subjected to heat treatment, the over-aging section may be used as a slow-cooling section.
The product quality is damaged if the surfaces of steel strips are oxidized during annealing treatment. Therefore, in a continuous heat-treatment furnace, the furnace is generally filled with a non-oxidizing atmospheric gas by providing gas feed channels 22a to 22f and gas discharge channels 23a to 23g in the individual treatment sections. A mixed gas (HN gas) of hydrogen gas and nitrogen gas has been used generally as the atmospheric gas in the continuous annealing furnace.
In the continuous heat-treatment furnace, different atmospheric gases may be used in adjacent treatment sections in the heating zone or in the cooling zone when heat treatment is performed.
For example, with an aim of improving the aging of low carbon steels, a method is disclosed in Japanese Examined Patent Publication No. 55-1969 or Japanese Unexamined Patent Publication No. 6-346156 in which, in order to increase the cooling rate in the rapid-cooling section before over-aging treatment, the heat-transfer coefficient is improved by increasing the hydrogen content in a cooling gas that is cyclically used in the rapid-cooling section.
In accordance with the processes disclosed in the above patent publications, if an atmospheric gas having a high hydrogen content in the rapid-cooling section leaks into the adjacent slow-cooling section or into the over-aging section, a large amount of hydrogen gas must be fed so that the high hydrogen content in the rapid-cooling section is maintained. Therefore, there is a need for a sealing apparatus which avoids mixture of atmospheric gases having different compositions between the rapid-cooling section and the slow-cooling section and between the rapid-cooling section and the over-aging section.
FIG. 4 is a schematic diagram showing an example of a seal roll apparatus which has been conventionally used for shutting off atmospheric gases in a continuous heat-treatment furnace. As shown in FIG. 4, seal rolls 24a and 24b are disposed so as to be opposed to each other with a steel strip S therebetween, and a partition 25 is disposed in the vicinity of the seal rolls in order to improve the sealing properties. The steel strip S passes through a space (hereinafter referred to as xe2x80x9ca roll gapxe2x80x9d) between the seal rolls 24a and 24b. The roll gap is adjusted to be as small as possible to improve the sealing properties, and each seal roll rotates (driven by motors M) so that scratches do not occur on the surfaces of the steel strips even when the travelling steel strip and the rolls are in contact with each other.
As a means for sealing atmospheric gases, an alternative to the seal roll apparatus described above, for example, a bulkhead structure is disclosed in Japanese Unexamined Patent Publication No. 5-125451. The bulkhead structure is disposed at the boundaries between atmospheric gases having different compositions and also functions as a plurality of treatment rooms enabling the feeding and discharging of atmospheric gases having different compositions. A seal roll apparatus is disclosed in Japanese Examined Utility Model Publication No. 63-19316 in which sealing members are disposed so as to be in contact with both surfaces of a steel strip. In Japanese Unexamined Patent Publication No. 59-133330, an apparatus is disclosed in which seal rolls, blowing nozzles, and seal dampers are combined.
However, there are problems in the conventional sealing apparatus as follows.
Although the contact-type sealing apparatus have excellent sealing properties, scratches may occur on the surfaces of the metal strips.
With respect to the gas sealing apparatus, although scratches do not occur on the surfaces of the metal strips because of the non-contact sealing structure, since the sealing gas must always be fed at a certain rate, the consumption of gas is increased. Additionally, in order to maintain sealing properties, the gas feeding must be controlled with high precision, resulting in a high cost of equipment.
In seal roll apparatus in which a steel strip is brought into contact with rotating seal rolls, such drawbacks associated with the above apparatus are not observed. Therefore, such seal roll apparatus are advantageous for practical use. However, since thermal expansion of seal rolls is unavoidable due to radiation heat from the steel strips or furnace walls or due to convective heat transfer through the atmospheric gases, there is a limit to narrowing the roll gap between the seal rolls, and thus atmospheric gases are not sufficiently shut off.
FIGS. 5A through 5C schematically show a seal roll apparatus in which a steel strip S is brought into contact with rotating seal rolls. When seal rolls 24a and 24b are subjected to radiation heat from the high-temperature steel strip S undergoing annealing and from the furnace wall, thermal expansion having a non-uniform temperature profile (hereinafter referred to as xe2x80x9ca thermal crownxe2x80x9d) is caused in the roll barrel direction of the seal rolls 24a and 24b. Thus, it is difficult to maintain a minimum gap between the seal rolls 24a and 24b in the roll barrel direction.
In order to eliminate the effect of the thermal crown, a certain distance must be maintained between the seal rolls and a partition 25 for separating the individual reaction sections.
Moreover, when the roll gap between seal rolls is narrowed, the seal rolls are operated in an attempt to prevent scratches from occurring even if a steel strip and the seal rolls are brought into contact with each other. However, unless the peripheral velocity of the seal rolls and the conveying velocity of the steel strip coincide with each other, scratches unavoidably occur on the surfaces of the steel strip.
The probability of scratches increases as the roll gap between seal rolls is decreased and the opportunity for contact between the seal rolls and the steel strip increases.
Normally, the peripheral velocity of the seal rolls is set to be equal to the value obtained by multiplying a measured rotational frequency of a conveyor roll by a circumference calculated from the diameter of the conveyor roll that has been preliminarily input to a controller. However, since the hearth conveyor roll in the heating zone is subjected to a high-temperature atmosphere, the actual roll diameter is larger than the roll diameter preliminarily input to the controller due to thermal expansion. Therefore, the actual conveying velocity of the steel strip is faster than the set peripheral velocity of seal rolls. The difference between the set seal roll peripheral velocity and the steel-strip conveying velocity increases as the rotational frequency of the hearth conveyor roll is increased, that is, as the conveying velocity is increased.
In accordance with one aspect of the present invention, a seal roll apparatus that hermetically seals boundaries between a plurality of heat-treating sections for continuously heating and cooling a strip material in a continuous heat-treatment furnace, is provided with at least a pair of water-cooled seal rolls opposed to each other with a gap therebetween for passing the strip material. Preferably, the water-cooled seal rolls are provided in a seal room defined by partitions having an opening for passing the strip material, a partition being provided in each of the inlet side and the outlet side of the seal rolls.
Water-cooling the seal rolls enables the temperatures of the seal rolls to be controlled, enabling them to be placed closer together. The pair of partitions shield the seal rolls from being directly subjected to radiation heat from heat sources and the furnace walls, thus further improving the ability to control the temperature of the seal rolls. This effect is further increased when the partitions are water-cooled.
Consequently, the sealing properties can be improved without causing deterioration in the quality, such as scratches, on the surfaces of the passing strip material.
Moreover, since the seal roll peripheral velocity preferably is set based on the measured surface temperature of a hearth conveyor roll placed very close to the seal rolls, the difference in velocity between the peripheral velocity of the seal rolls and the conveying velocity of the strip material can be reduced substantially to zero, thus avoiding the occurrence of scratches which may lead to deterioration in quality.